1. Field of the Invention
The present invention relates to an external cavity semiconductor laser light source which is used in any technical field which requires a light signal source, in particular, which is used in an optical telecommunication or a coherent optical measuring technical field.
2. Description of the Related Art
In the coherent optical measuring technical field, an external cavity semiconductor laser (hereinafter, may be simply referred to LD) is generally used as a light source unit in a light source tunable in wavelength. An optical filter which is a wavelength selection element is disposed in the external resonator thereof to get a single mode oscillation.
Such a technique enables wavelength scanning in a wide range by mechanically varying the transmitted (or reflected) wavelength from the optical filter.
In an optical frequency stabilized member, an optical PLL (phase-looked loop) for feeding back an electric current into an LD driving current, which is proportional to a difference in frequency between an output light and a reference light is generally used.
FIG. 5 shows a construction example of an optical frequency stabilized external type of LD light source according to an earlier development. In FIG. 5, the reference numeral 1 denotes an external cavity LD light source unit, 2 denotes an optical filter, 3 denotes a first driving unit (drive unit 1), 4 denotes a wavelength control unit, 5 denotes an optical branching unit, 6 denotes an optical frequency/voltage conversion unit, 7 denotes an optical frequency reference light source, 8 denotes a first low pass filter (LPF1) and 9 denotes an amplifier.
First, the basic operation of the external cavity LD light source will be explained.
The first drive unit 3 can mechanically change wavelength of the transmitted beam or of the reflected beam, from the optical filter 2, and when occasion demands, it serves as a wavelength scanning mechanism for carrying out also adjustment for the length of an external resonator.
The relationship between the state of the first drive unit 3 and the oscillation wavelength of the external resonator is already known. In the wavelength control unit 4, a wavelength set can be carried out with the wavelength resolution of about 1 pm on the basis of a formula showing the known relationship. When occasion demands, the wavelength control unit 4 also controls the LD drive current in order to control the optical output level of the LD light source unit 1.
FIG. 6 shows a construction example of an external cavity LD light source unit 1. In FIG. 6, the reference numeral 101 denotes a diffraction grating; 102, 105 and 107 denote lenses; 103 denotes an anti-reflection film; 104 denotes an LD; 106 denotes an optical isolator; 108 denotes an optical fiber; and 109 denotes an LD driving circuit.
In the external cavity LD light source unit 1 shown in FIG. 6, the diffraction grating 101 which corresponds to the above-described optical filter 2, as shown in FIG. 5, also functions as a mirror in one side of the external resonator.
That is, the external resonator is formed by the end surface B of the LD and the diffraction grating 101. The length of the resonator is defined by the segment AB of a line from the end surface B of the LD to the point A at which the grating surface of the diffraction grating 101 intersects the optical axis X of the diffraction grating 101.
An anti-reflection film 103 is formed on an end surface in the side of the diffraction grating 101, of the LD 104 in order to remove unnecessary reflection.
Each of the lenses 102 and 105 is a collimator for changing the output beam of the LD 104 to a collimated beam.
The output beam from the external resonator LD 104, which is obtained from the side of the LD end surface B, is condensed through the lens 107 and is taken out by the optical fiber 108.
In order to generate no noise due to external feedback beam from a following optical system, the optical isolator 106 is disposed on the way in the output side.
The LD driving circuit 109 feeds an LD driving current corresponding to a desired optical output level, and is controlled by the wavelength control unit 4 shown in FIG. 5, as described above.
Although only the diffraction grating 101 is shown in FIG. 6, as the above-described optical filter 2 shown in FIG. 5, it is possible to use an optical element other than the diffraction grating 101, for example, to use an interference filter or the like, as the optical filter 2. When occasion demands, combination of a plurality of optical elements can be also used as the optical filter 2.
Next, the characteristics of an optical filter will be explained as follows.
FIG. 7 shows an optical filter of the diffraction grating 101. In this Figure, .theta. is the angle of the normal N.sub.gr to the diffraction grating 101 with the optical axis, d is the pitch of the diffraction grating, and the incident light and the reflected (diffracted) light are set on the same optical axis X, like the above-described external resonator shown in FIG. 5.
The spectrum of the reflected light when an incident white light came into the diffraction grating 101 is the filter characteristics of the diffraction grating. A filter characteristics is obtained, as shown in FIG. 8.
The reflection peak wavelength .lambda..sub.gr is given by the following Bragg's formula: EQU .lambda..sub.gr =2 d.times.sin (.theta.) (1)
The characteristics of the interference filter 201 in the optical system shown in FIG. 9 has periodic transmittance peaks, as shown in FIG. 10.
In FIG. 9, "D" is the thickness of the interference filter 201, and "n" is the refractive index thereof.
The wavelength of each transmittance peak is given by the following formula: EQU k.lambda..sub.k =2nD.times.cos (.phi.) (2)
where .phi. is the angle of the normal N to the interference filter 201 with the optical axis in the interference filter 201, and k is an integer.
When L is the length of the external resonator and m is an integer, like the case of an interference filter, the oscillation longitudinal mode wavelength .lambda..sub.m of the external resonator is expressed as follows: EQU m.lambda..sub.m =2 L (3)
Next, an example of characteristics of wavelength tunable LD light source, using an optical filter comprising a combination of a diffraction grating and an interference filter is illustrated in FIGS. 11A to 11E.
FIG. 11A shows a gain characteristics of LD, which generally has a gain in a wavelength range not less than 100 nm.
FIG. 11B shows a resonator mode corresponding to the formula (3), which are oscillation longitudinal modes.
Several modes are selected by using the filter characteristics of the diffraction grating 101, corresponding to the formula (1), which is shown in FIG. 11C among the oscillation modes.
Then, a signal mode is selected by using the filter characteristics of the interference filter, corresponding to the formula (2), which is shown in FIG. 11D. As a result, a single mode oscillation is obtained, as shown in FIG. 11E.
By changing each of the characteristics shown in FIGS. 11B, 11C and 11D, that is, by changing L, .theta. and .phi., suitably, it is possible to carry out wavelength scanning.
The relationship between the wavelength and L, .theta. and .phi. is found on the basis of previous measurements. According to the relationship, the first drive unit 3 having a combination of a motor, a rotary table, a directly linear-moving mechanism and the like realizes a state of particular values of L, .theta. and .phi., corresponding to the set wavelength which is set at intervals of 1 pm.
The above descriptions are the explanations of the basic operation of the external cavity LD light source.
Next, an optical frequency stabilizing method will be explained below.
In the optical frequency stabilizing method which is described below, an optical PLL which is well-known as an optical frequency controlling method is used.
In FIG. 5, an output beam of the external cavity LD light source unit 1 is branched by an optical branching unit 5. One of the branched output beams is an optical output to be outputted from the external cavity LD light source, and the other is one incident light of an optical frequency/voltage conversion unit 6. The optical frequency thereof is "f".
The optical frequency of the other incident light of the optical frequency/voltage conversion unit 6, which is outputted from an optical frequency reference light source 7 is "f.sub.1 ".
The optical frequency/voltage conversion unit 6 generates an error signal of which output is proportional to a difference in optical frequency between two incident lights (f-f.sub.1). The high-frequency component thereof is removed by a first low pass filter 8, and after amplified by an amplifier 9, the error signal is fed back into an LD driving current.
Because the change of the LD driving current causes the change of the length L of an external resonator, described in the formula (3), the optical frequency "f" is changed. The optical PLL is formed by setting the characteristics of the feedback loop of the optical PLL so that (f-f.sub.1) makes zero, that is, so that the optical frequency "f" is changed close to "f.sub.1 ".
In the feedback control, the method of feeding an electric current into a phase adjustment region, or the like, is adopted other than that of using the LD driving current. However, because the method is equivalent to that of using the LD driving current in point of changing the length of an external resonator, only the case of using the LD driving current will be explained below.
FIG. 12 shows an example of a value of an output error signal with respect to a difference in optical frequency between two incident lights received by the optical frequency/voltage conversion unit 6. When the difference .vertline.f-f.sub.1 .vertline. is larger than f.sub.max (.vertline.f-f.sub.1 .vertline.&gt;f.sub.max), the error signal has a constant value, so that the operation of the optical PLL is not carried out. By previously setting the difference in optical frequency between two incident light by the wavelength control unit 4 so that the error signal is within the range of .+-.V.sub.max, the operation of the optical PLL is carried out.
FIG. 13 shows a construction example of the optical frequency/voltage conversion unit 6.
In FIG. 13, the reference numeral 61 denotes an optical multiplexer, 62 denotes a photoelectric conversion unit, 63 denotes a high pass filter (HPF), 64 denotes a frequency/voltage conversion unit and 65 denotes a reference electrical signal source.
The other branched beam (optical frequency f) of the optical branching unit 5 is multiplexed with the output light (optical frequency f.sub.1) of the optical frequency reference light signal 7 by the optical multiplexer 61, and then the multiplexed light is received by the photoelectric conversion unit 62.
The photoelectric conversion 62 corresponds to the optical frequency/voltage conversion unit 6, however, the output of the output signal thereof is proportional to the absolute value of the difference in optical frequency .vertline.f-f.sub.1 .vertline. as shown in FIG. 14. From this reason, when the relationship between the optical frequencies of the lights received by the optical multiplexer 61 is reversed, the photoelectric conversion unit 62 cannot control the signal to be outputted to the HPF 63.
In order to avoid this problem, an offset lock is carried out so that, for example, the difference in optical frequency (f-f.sub.1) can make f.sub.2. It is required that the offset lock frequency f.sub.2 is set to be high enough as compared with a change of the frequency of the received light.
The direct current component of the output from the photoelectric conversion unit 62 is removed by the high pass filter (HPF) 63, and then only the absolute value of the difference in frequency .vertline.f-f.sub.1 .vertline. is outputted to the frequency/voltage conversion unit 64.
The frequency of the electrical signal outputted from the reference electrical signal source 65 which generates a reference electrical signal is the offset frequency f.sub.2.
The frequency/voltage conversion unit 64 into which the output signal of the high pass filter (HPF) 63 and the reference signal from the reference electrical signal source 65 are inputted, generates an error signal of which output is proportional to the difference in frequency of the two input signals (.vertline.f-f.sub.1 .vertline.-f.sub.2). For example, when the frequency f is higher than f.sub.1, the frequency/voltage conversion unit 64 generates the signal of which output is proportional to the difference (f-f.sub.1 -f.sub.2).
By the construction, the optical frequency f can be stabilized with (f.sub.1 +f.sub.2).
Next, FIG. 15 shows another construction example of the optical frequency/voltage conversion unit 6 and the optical frequency reference light source 7.
In FIG. 15, the reference numeral 66 denotes a Fabry-Perot interferometer.
The other branched light of the optical branching unit 5 is transmitted through the Fabry-Perot interferometer 66, and is converted into an error signal by the photoelectric conversion unit 62.
FIGS. 16A and 16B show a transmittance characteristics of the Fabry-Perot interferometer 66, and an error signal characteristics of the photoelectric conversion unit 62.
The transmittance characteristics of the Fabry-Perot interferometer 66 has peak wavelengths in the multimode according to the formula (2), similarly to the FIG. 11D.
FIG. 16A shows a transmittance characteristics of the Fabry-Perot interferometer 66 in the mode k.
In this case, a transmittance A of light having the optical frequency f.sub.1 is used as a reference corresponding to the optical frequency reference signal 7.
FIG. 16B shows an error signal characteristics of the photoelectric conversion unit 62.
The locking range for locking the mode in which the designated optical frequency can be obtained, has a range of up to about f.sub.max, in which the output of the error signal is approximately proportional to the difference in optical frequency (f-f.sub.1).
Because the optical frequency reference value f.sub.1 exists in each mode clearly according to the formula (2), the desirable mode in which the designated frequency can be obtained can be selected and can be locked.
However, in the case of the external cavity LD light source, the optical frequency is largely drifted because a change with the passage of time is caused, in particular, by a fluctuation in temperature. From this reason, the operation of the optical PLL is influenced by a change with the passage of time.
FIGS. 17A to 17C show each characteristics of the optical frequency stabilized external cavity LD light source according to an earlier development.
FIG. 17A shows a change of a DC component of an output from the first low pass filter 8 with the passage of time, FIG. 17B shows a change of LD driving current with the passage of time and FIG. 17C shows a change of the optical frequency f with the passage of time.
After the operation of the optical PLL started at the time t.sub.1, by drifting the output from the first low pass filter 8 in accordance with a drift of the optical frequency f due to a change with the passage of time, the optical frequency f is kept constant. At the same time, because the LD driving current is drifted, the output of the output beam which is approximately proportional to the LD driving current is also drifted.
However, after the time t.sub.2 when the DC component of the output from the first low pass filter 8 exceeded V.sub.max, the operation of the optical PLL cannot be carried out, and the drift of the optical frequency occurs with a free run state.
The LD driving current becomes the maximum value (I.sub.d +.DELTA.), where .DELTA. is the maximum feedback current corresponding to V.sub.max, which is amplified by the amplifier 9. Then, the LD driving current is not changed.
Further, because the change of the LD driving current causes only the change of the length L of the external resonator, as described above, the characteristics of the optical filter is not changed by the change of the LD driving current. Therefore, according to FIGS. 11B, 11C and 11D, because the characteristics of the optical filters continue to be drifted contrary to the resonator mode locked to f.sub.1 at the operation of the optical PLL, an optical frequency gap between f.sub.1 and a maximum transmittance rate point (or a maximum reflection rate point) of the optical filter 2 occurs.
Then, when the maximum optical frequency gap extends from mode to mode, the oscillation mode is transited to an adjacent resonator mode, that is, a mode hop occurs.
FIGS. 18A to 18C show characteristics of the optical frequency stabilized external resonator LD light source according to an earlier development in the case of causing a mode hop.
According to the characteristics shown in FIGS. 18A to 18C, the operation of the optical PLL starts at the time t.sub.1, similarly to those shown in FIGS. 17A to 17C. However, at the time t.sub.3 when the DC component of the output from the first low pass filter 8 is lower than V.sub.max, a mode hop occurs.
That is, because the optical frequency is changed up to (f.sub.1 +c/(2 L)) in an instant, the output from the first low pass filter 8 reaches a saturation point V.sub.max. As a result, the operation of the optical PLL cannot be carried out, and then the optical frequency is in a free run state. In this optical frequency (f.sub.1 +c/(2 L)), the term c/(2 L) denotes a resonator mode interval and the character c denotes a light velocity.
Because the resonator mode interval of the external cavity LD light source is about 5 GHz in general, in order to assure a normal operation of the optical PLL even though a mode hop occurs, it is required to secure the f.sub.2 and f.sub.max having at least over 5 GHz frequency.
The change of the optical frequency with respect to the LD driving current is about 1/10 as compared with an LD only, that is, about 0.1 GHz/mA.
Therefore, it is required that the LD driving current for compensating 5 GHz optical frequency gap occurring at a mode hop is about 50 mA. In consideration of the LD driving current of about 100 mA in a normal state, there is possibility that a large change of the optical output occurs.